Lithium ion battery is an ideal power source for portable electronic equipments, electric vehicles, energy storage systems. It is the core of the research and development field of the lithium ion battery to develop a novel electrode material having high specific energy, better safety and low cost. Studies on novel negative electrode materials have an important meaning to the development of new-generation lithium ion battery. The negative electrode materials of the current mature lithium ion battery are mainly graphite materials having a theoretical specific capacity of 372 mAh/g and limited developing potential, and thus cannot meet the demand of the future lithium ion battery on high-energy density. It is found by studies that the reversible lithium storage capacity of the metals, such as Al, Sn, Sb, Si and the like which can be alloyed to Li, and other alloy materials, is far greater than graphite negative electrode. For example, the theoretical capacity of Si negative electrode is as high as 4200 mAh/g. However, such negative electrode material has a greater lithiation/delithiation volume expansion and shrinkage (>300%), and high volume effect renders worse cycle stability, resulting in that there is a certain distance between these systems and practical level.
In order to increase the cycle performance of the silicon negative electrode, those skilled in the art would generally improve the volume expansion effect of the silicon material by the nano-crystallization of silicon, alloying of silicon with metals, and complexing of silicon and carbon materials, wherein the composite material of nano-silicon and graphite has a greater application prospect; the synthesis of nano-silicon and the homogeneous dispersion thereof in graphite matrix are the key technologies.
CN103474667A discloses a silicon-carbon composite negative electrode material, comprising nano-silicon/graphite particles, first carbon coating layer and organic cracking carbon layer, wherein the nano-silicon/graphite particles are spherical or spherical-like composite particles formed by coating graphite as inner core with nano-silicon particle layer. CN100422112C discloses a silicon-carbon composite material having a spherical core-shell structure, wherein the inner core part of the core-shell is spherical carbon particles having an average particle size of 1-45 micrometer; the carbon particles are one or a mixture of two or three materials selected from the group consisting of graphitized intermediate phase carbon globules, hard carbon globule and spherical graphite; the shell layer of the core-shell is composed of carbon and silicon crystal particles having an average particle size of from 10 nanometer to 4 micrometer. The aforesaid structures both involve coating nano-silicon onto the surface of graphite particles, which easily result in uneven distribution of the nano-silicon, being unable to effectively use the space inside the graphite particles, and being unable to further increase the loading capacity of the nano-silicon.
The hollowing treatment of graphite will be helpful to embedding nano-silicon into the inside of graphite particles, so as to effectively increase the dispersibility and loading capacity of the nano-silicon.
CN103682287A discloses a silicon-based negative electrode material embedded with a composite core-shell structure, wherein the inner core is a structure formed by embedding nano-silicon particles into the inner voids of hollowed graphite; and the shell is non-graphite carbon material; and the inner core is obtained by mixing and drying the nano-silicon obtained by mechanical grinding with the hollowed graphite in organic solvent. However, the nano-silicon particles are very easy to agglomerate during the drying, and thus cannot form the very homogeneous and monodispersed composite state of the nano-silicon particles inside the hollowed graphite. The agglomerated nano-silicon will gradually fuse during the charge and discharge process to form large-grained silicon, and the cycle and expansion performance will notably degrade. CN102214817A discloses a carbon/silicon/carbon nano composite negative electrode material and process for preparing the same, comprising depositing nano-silicon on carbon matrix by chemical vapor deposition, and then coating nano-carbon onto the surface of the nano-silicon by chemical vapor deposition. The carbon matrix material is selected from the group consisting of porous carbon, carbon nanotube and graphene. However, common porous carbon material (e.g. activated carbon) has too small voids to effectively support nano-silicon material. The carbon nanotube or graphene itself is easy to agglomerate, and thus cannot achieve homogeneous deposition of nano-silicon material on the surface thereof.
Thus it is a technical problem to be solved in the lithium ion battery field that how to prepare nano-scale silicon particles, to achieve homogeneous and monodispersed loading of nano-silicon inside graphite particles, to effectively reduce the cycle expansion of the material while ensuring high capacity, and to increase cycle performance, so as to prepare silicon-based negative electrode material having high capacity and long service life.